The Mets’ season of injuries continued with an unexpected turn on Thursday. Jeurys Familia, the Mets closer who struggled with his command in blowing a save against the San Francisco Giants on Wednesday, was found to have an arterial clot in his throwing shoulder, the team announced.

It was an ominous-sounding ailment that could jeopardize Familia’s season, and a surprising announcement considering how optimistic Familia had sounded after the loss to the Giants.

“I feel great,” he said after the game, a 6-5 loss. “My arm feels good. I just don’t have it today.”

But something was indeed wrong with his arm. The Mets said Familia had been examined in New York by team doctors, who discovered the clot. Familia, 27, will see Dr. Robert Thompson in St. Louis on Friday for further tests and possibly surgery, the team said. Thompson performed season-ending surgery to alleviate thoracic outlet syndrome on Mets starter Matt Harvey last July. Harvey had a rib removed to ease pressure on a nerve that was causing numbness in his fingers.

The latest revelation would presumably send Familia to the disabled list, where he would join, among other key Mets, outfielder Yoenis Cespedes, infielder David Wright, catcher Travis d’Arnaud and pitchers Noah Syndergaard, Seth Lugo and Steven Matz.

The Mets have stayed afloat without the bedrocks of their lineup (Cespedes) and their rotation (Syndergaard), and now they have lost the anchor of their bullpen in Familia. They have won eight of their past 12 games, improving their record to 16-17, and surged into second place in the National League East thanks to an improved offense and bullpen.

But how many more blows can the Mets take this season and still stand?

The Mets were mum on Familia’s prognosis on Thursday, an off day. General Manager Sandy Alderson was expected to address the situation on Friday afternoon before the team’s game in Milwaukee.

Thursday’s announcement caught many off guard because Familia had appeared to find a groove recently after initially struggling with his command following his return three weeks ago from a 15-game domestic-abuse suspension. Before Wednesday, his previous five outings were clean, and he notched three saves.

There may have been small signs that something was not right. The vertical release point of the ball in Familia’s delivery improved over the past few weeks after having started the season a few inches lower compared with the previous three years, according to the pitching analytics website BrooksBaseball.net. Altered mechanics are sometimes a sign of physical changes.

Asked after Wednesday’s game if he felt he had fixed the issues that had previously been hurting his command, Familia said: “The command is getting better. I feel good.”

Other pitchers have recovered from blood clots in the arm. The former Mets pitcher Dillon Gee, after a strong outing in July 2012, complained of numbness in his fingers the next day. He had surgery to remove a blood clot in his pitching shoulder a day later. Gee missed the rest of the season and had thoracic outlet syndrome surgery last year.

David Cone and Ian Kennedy are among those who have pitched well after the removal of blood clots. Tom Glavine treated his with blood thinners.

The possible causes of a blood clot include trauma, repetitive motion or the buildup of scar tissue, said Dr. Allan Stewart, the medical director of aortic surgery at Mount Sinai Health System in New York.

“If it’s truly arterial, it’s generally going to be treated with surgery,” said Stewart, who was not involved in Familia’s treatment.

Familia had the most appearances of any reliever in baseball from 2014 to 2016, with 230. He posted a 2.20 earned run average in that span and logged at least 77 innings in each of those seasons, a model of consistency but also a heavy workload that caused some concern for the Mets.

Last season, Familia earned his first All-Star Game selection and logged a major-league leading 51 saves. He coughed up the only runs, however, in the Mets’ 3-0 loss to the Giants in the N.L.’s wild-card playoff game.

In Familia’s place, the Mets will most likely turn back to the setup man Addison Reed, a former closer. Reed was perfect in four save chances as the Mets’ closer during Familia’s suspension.

A version of this article appears in print on , on Page B13 of the New York edition with the headline: Mets Add Familia To Long List Of Injured. Order Reprints | Today’s Paper | Subscribe